1. Field of the Invention
The present invention relates to a transmission for a hydraulically driven vehicle, and more particularly, to an automatic transmission for a hydraulic excavator or the like which has an auxiliary speed change gear capable of shifting between high and low speed ranges.
2. Description of the Related Art
In a conventional hydraulic excavator or the like which has an upper swivel body and a lower travel body and which runs with an hydraulically driven motor provided with an auxiliary speed change gear capable of changing between high and low speed ranges, the auxiliary speed change gear is of constant-mesh type and is manually operated by means of an operation lever to change speeds between high and low speeds. Speed change operation therefore requires that the vehicle be stopped temporarily, and is thus troublesome: it is difficult to conduct speed change operation quickly and smoothly.
An auxiliary speed change gear which employs a wet multiple disk clutch hydraulically operated to change between high and low speed ranges by means of an operation valve has also been proposed in, for example, Japanese Utility Model Laid-Open No. 172817/1986. With this auxiliary speed change gear, an operation of shifting a speed changing gear from low to high speed, which is conducted while the vehicle is running at a high speed, may cause overrun of the hydraulically driven motor and damage the hydraulic motor or a low speed clutch.